Vector rendering with the DOM renderer

[ahocevar]

This change makes vector rendering work with the DOM renderer. It includes support for vector layers, feature overlays and custom rendering using precompose/render/postcompose events. A 2D canvas is used for rendering vectors with the DOM map renderer.

[ahocevar]

Work in progress. As discussed with Eric and Tim, I will change this pull request so vector layers can be rendered with the DOM renderer.

[elemoine]

@ahocevar cool. I'm looking forward to seeing the DOM renderer draw vector layers using Canvas. Note that there already is a case where the canvas vector layer renderer uses its own (intermediate) canvas. See https://github.com/openlayers/ol3/blob/master/src/ol/renderer/canvas/canvasvectorlayerrenderer.js#L87-L92. This is used in the case of heatmap layers.

Also, I have a branch that makes it possible to bind a feature overlay to a layer (of any type): https://github.com/elemoine/ol3/compare/featureoverlay. So with this you can use a feature overlay to draw features above a given layer. This is quite similar to this PR's current branch.

[ahocevar]

This is now ready for review.

[elemoine]

This looks good to me. +1 for merging this.